Small vertical-take-off air vehicles, sometimes called drones, are used for recreation and commercial purposes such as inexpensive aerial photography. Typically these air vehicles rely on a set of rotating blades for both propulsion and lift. These vehicles are usually powered by batteries. Decreasing the power consumption of the vehicle permits longer flight times with a given battery size or increased payload by using a smaller battery. Larger vehicles, such as manned air vehicles and near-surface vehicles such as hovercraft also benefit from efficient aerodynamic propulsion, lift, or both. Similarly, water vehicles such as submarines or boats may benefit from more efficient hydrodynamic propulsion.
An aerodynamic or hydrodynamic rotor typically has airfoil-shaped blades designed to propel air or water parallel to the axis of rotation. The surfaces of the blades are predominantly normal to the axis of rotation, although they may be inclined slightly to provide some angle of attack and may be twisted such that the incidence is higher near the axis of rotation. Impellers, on the other hand, have a series of vanes oriented in a plane that is nearly parallel to the axis of rotation. Impeller vanes do not need an airfoil cross section. Impeller vanes propel air in a circumferential direction around the axis of rotation. The circumferential movement causes centripetal forces which propel the air in a radial direction away from the axis of rotation while also generating a vortex pattern.